... Either way, any clustering process requires calculating the distance of all points (not between all the points, but of all of them to some relative point). Because i'll need a clustering MR job, ill probably use it, despite as you said, it has high probability to be correct (not 100%)...

This is probably right as stated, but I think that there is confusion here.

Many people assume that each point in the training data has to have distance computed to all centroids in the clustering. Even this is not true.

It is true that you have to compute distance to at least one something, but not necessarily to all of the clusters.